I'm using a 4GB SD card with Raspbian "Jessie" Lite to boot, with a 16GB usb drive on an internal port for storage. The Pi is actually powered through the GPIO pins, which are connected to 5V and ground on P4 on the PSU (old floppy drive power plug). The speakers are run off of 5V as well, coming from P6 (P1 extender [12V, 5V, ground]), going into the battery terminals. The speaker could take either four AA batteries (4.8V-6V) or a 9V plug, so splicing the battery wires was easier.

I'm actually considering chaining some "Raspberry Pi Zero"s onto it to add some extra kick, because they're only $5 a piece now, and pack a 1GHz CPU with 512MB RAM. Also, because they're so small, I can easily fit them inside my existing project. Can't say I've ever tried "cluster computing," but it'd be fun to try.